Sometimes the rotor is a little out of true and the shield is just barely rubbing on the high spots. I've seen that many times. Those shields take a beating sometimes when you're breaking ball joints loose and such.

Another thing I've seen from time to time happens when the rotors aren't replaced or turned. Sometimes the new pads are a slightly different size or sit in a slightly different position in the caliper and rub on the rust that builds up at the edge of the contact surface. Usually it goes away within a day or so.